In recent years, industrial and utility concerns with deregulation and operational costs have strengthened demands for increased power plant efficiency. The Rankine cycle power plant, which typically utilizes water as the working fluid, has been the mainstay for the utility and industrial power industry for the last 150 years. In a Rankine cycle power plant, heat energy is converted into electrical energy by heating a working fluid flowing through tubular walls, commonly referred to as waterwalls, to form a vapor, e.g. turning water into steam. Typically, the vapor will be superheated to form a high pressure vapor, e.g., superheated steam. The high pressure vapor is used to power a turbine/generator to generate electricity.
Conventional Rankine cycle power generation systems can be of various types, including direct-fired, fluidized bed and waste-heat type systems. In direct fired and fluidized bed type systems, combustion process heat is generated by burning fuel to heat the combustion air which in turn heats the working fluid circulating through the systems' waterwalls. In direct-fired Rankine cycle power generation systems the fuel, commonly pulverized-coal, gas or oil, is ignited in burners located in the waterwalls. In bubbling fluidized bed Rankine cycle power generation systems pulverized-coal is ignited in a set in a bed located at the base of the boiler to generate combustion process heat. Waste-heat Rankine cycle power generation systems rely on heat generated in another process, e.g., incineration, for process heat to vaporize, and if desired superheat, the working fluid. Due to the metallurgical limitations, the highest temperature of the superheated steam does not normally exceed 1050.degree. F. (566.degree. C.). However, in some "aggressive" designs, this temperature can be as high as 1100.degree. F. (593.degree. C.).
Waterwalls are formed of tubes which serve as flow passages for the working fluid. Hence, the waterwalls must be capable of being subjected to the pressure loads generated by the working fluid throughout the cycle. Typically, the waterwalls must also be capable of being subjected to other loads. For example, in many cases the waterwalls must be self supporting. It is also common for the waterwalls to have mounted in supported relation thereon other system elements, such as burners, a lower drum, and/or sootblowers. Accordingly, it is important that the waterwalls have the structural integrity to withstand the required loadings throughout a desired design life.
The waterwall tubes are conventionally made of steel. In a typical Rankine cycle power system, the waterwall tubes in those portions of the system which are subjected to lower temperatures may be of one type of steel while the waterwall tubes in higher temperature portions of the system are of a different type steel. Thus, the waterwall tubes in the lower temperature areas may be formed of low alloy steel, commonly referred to as ferritic steel, for example, having 21/2Cr to 16Cr. Waterwall tubes in the higher temperature areas may be formed of high alloy steel, commonly referred to as austenitic or stainless steel, for example having 18Cr and 8Ni.
Over the years, efficiency gains in Rankine cycle power systems have been achieved through technological improvements which have allowed working fluid temperatures and pressures to increase and exhaust gas temperatures and pressures to decrease. An important factor in the efficiency of the heat transfer is the average temperature of the working fluid during the transfer of heat from the heat source. If the temperature of the working fluid is significantly lower than the temperature of the available heat source, the efficiency of the cycle will be significantly reduced. This effect, to some extent, explains the difficulty in achieving further gains in efficiency in conventional, Rankine cycle-based, power plants.
In view of the above, a departure from the Rankine cycle has recently been proposed. The proposed new cycle, commonly referred to as the Kalina cycle, attempts to exploit the additional degree of freedom available when using a binary fluid, more particularly an ammonia/water mixture, as the working fluid. The Kalina cycle is described in the paper entitled: "Kalina Cycle System Advancements for Direct Fired Power Generation", co-authored by Michael J. Davidson and Lawrence J. Peletz, Jr., and published by Combustion Engineering, Inc. of Windsor, Conn. Efficiency gains are obtained in the Kalina cycle plant by reducing the energy losses during the conversion of heat energy into electrical output.
A simplified conventional direct-fired Kalina cycle power generation system is illustrated in FIG. 1 of the drawings. Kalina cycle power plants are characterized by three basic system elements, the Distillation and Condensation Subsystem (DCSS) 100, the Vapor Subsystem (VSS) 110 which includes the boiler 142, superheater 144 and recuperative heat exchanger (RHE) 140, and the turbine/generator subsystem (TGSS) 130. The boiler 142 is formed of tubular walls 142a and the superheater 144 is formed of tubular walls 144a. A heat source 120 provides process heat 121. A portion 123 of the process heat 121 is used to vaporize the working fluid in the boiler 142. Another portion 122 of the process heat 121 is used to superheat the vaporized working fluid in the superheater 144.
FIG. 1A depicts an expanded view of the boiler tubular walls 142a through which the working fluid flows. As shown, the tubular walls 142a are formed of steel tubes 150. As is customary in Rankine cycle power systems, the tubes 150 have milled inner surfaces 155. FIG. 1B depicts an expanded view of the superheater tubular walls 144a through which the vaporized working fluid flows. As shown, the tubular walls 144a are formed of steel tubes 160. The tubes 160 also have conventional milled inner surfaces 165. Those skilled in the art will recognize that the tubes similar to those forming the tubular walls are also utilized to transport the working fluid in other components of the VSS 110, the TGSS 130 and the DCSS 100.
During normal operation of the Kalina cycle power system of FIG. 1, the ammonia/water working fluid is fed to the boiler 142 from the RHE 140 by liquid stream FS 5 and by liquid stream FS 7 from the DCSS 100. The working fluid is vaporized, i.e. boiled, in the tubular walls 142a of the boiler 142. The vaporized working fluid from the boiler 142, along with working fluid vaporized in the RHE 140, is further heated in the tubular walls 144a of the superheater 144. The superheated vapor, identified as FS vapor 40 is directed to and powers the TGSS 130 so that electrical power 131 is generated to meet the load requirement.
The expanded working fluid FS extraction 11 egresses from the TGSS 130, e.g., from a low pressure (LP) turbine (not shown) within the TGSS 130, and is directed to the DCSS 100. The expanded working fluid is, in part, condensed in the DCSS 100. Condensed working fluid, as described above, forms feed stream FS 7 to the boiler 142. The DCSS 100 also separates the expanded working fluid into an ammonia rich working fluid flow FS rich 20 and an ammonia lean working fluid flow FS lean 30. Waste heat 101 from the DCSS 100 is dumped to a heat sink, such as a river or pond.
The rich and lean flows 20, 30 respectively, are fed to the RHE 140. Another somewhat less expanded hot working fluid FS extraction 10 egresses from the TGSS 130, e.g., from a high pressure (HP) turbine (not shown) within the TGSS 130, and is directed to the RHE 140. Heat transferred from the expanded working fluid FS extraction 10 to the rich flow FS rich 20, vaporizes the FS rich flow 20 and condenses, at least in part, the expanded working fluid stream FS extraction 10, in the RHE 140. The vaporized rich flow is fed to the superheater 144 along with vaporized feed fluid from the boiler 142. The condensed expanded working fluid forms part of the feed flow, i.e., flow FS 5, to the boiler 142, as has been previously described.
As discussed above, unlike Rankine cycle power systems which typically utilize water as the working fluid, Kalina cycle power generation systems utilize a mixture of ammonia and water as the working fluid. When materials, such as steel, are exposed to environments containing ammonia at a high temperature, such as the vaporized working fluid in a Kalina type power generation system, dissociation of the ammonia may occur due to the catalytic reaction at the surface of the material. Hence in a Kalina cycle power generation system, such a reaction can occur at the surface of the steel tubular walls of the boiler 142 and the superheater 144, as well as other areas of the system subjected to the binary working fluid. The nitrogen formed in this process may cause nitriding of the fluid walls. Materials subjected to nitriding are known to become brittle.
FIG. 2 depicts a cross section of a conventional superheater tube 160 of the superheater 144 of the FIG. 1 Kalina cycle power system. If the tube is formed of the austenitic steel having 18Cr and 8Ni, as in a conventional Rankine cycle superheater, the inner milled surface 165 forming the flow passage for the Kalina cycle system working fluid will, after being exposed to the ammonia/water working fluid will become degraded due to the nitriding. As will be understood by those skilled in the art, this tubular wall tubing degradation could result in the tubular walls having insufficient structural integrity to withstand the required loadings throughout the desired system design life, typically 20 years or more.
FIG. 3 depicts a cross-section of a conventional boiler tube 150 of the boiler 142 of the FIG. 1 Kalina cycle power system. The tube, as discussed above is formed of ferritic steel having 21/2-16Cr and 1Mo. Using conventional nitride testing procedures, the tube 150, after being exposed to the ammonia/water working fluid, the tube 150, has as shown in FIG. 3, a hardened layer 157 formed within the tube wall 159 due to nitriding. Hence, it is conventionally assumed that this fluid wall tubing degradation due to nitriding could result in the fluid walls becoming brittle and, therefore, having insufficient structural integrity to withstand the required loadings throughout the desired design life of the FIG. 1 power system.
The results of the conventional testing of the boiler tube 150 are shown in FIGS. 4A and 4B. More particularly, the depth and hardness of the nitride layer 157 shown in FIG. 3 are as shown in FIGS. 4A-B.
If the tube 150 is formed of a ferritic steel having a chromium content of 9%, as in a conventional Rankine cycle power system, and the tube is conventionally tested for nitriding, the expected hardness and hence brittleness of the nitride layer 157 of FIG. 3 will be much greater than 900 Vickers and the depth of the nitride layer after 1000 hours will be much greater than 100 microns. Such a hard nitride layer formed in the wall 159 of the tube 150 over such a substantial depth during such a short duration indicates that during a practical system design life, such nitriding would likely degrade the structural integrity of the tube such that it would be unable to withstand the required loading of the boiler walls over the design life.
If the chromium content of the tube 150 is reduced to 21/2%, as indicated in FIG. 4B, and the tube is conventionally tested, the hardness and hence brittleness of the nitride layer 157 shown in FIG. 3 is reduced but still remains well over 300 Vickers; however, the depth of the nitride layer after 1000 hours is substantially increased to well over 500 microns. Here again, such a hard nitride layer formed in the wall 159 of the tube 150 over such a substantial depth and formed during such a short period indicates that nitriding during a practical system design life would very likely result in such degradation to the structural integrity of the tube that it would be insufficient to withstand the required loading of the boiler walls over the desired design life. Accordingly, it is generally accepted, based upon conventional testing, that the nitriding of ferritic steel tubes 150 of the type conventionally used in Rankine cycle boilers, will be unsuitable for practical Kalina cycle boilers.
FIG. 5 depicts a cross-sectional view of a tube 350 formed of mild steel. As indicated in FIG. 5, when the mild steel tube 350 is tested conventionally at ambient pressure, a nitride layer 357 forms below the ammonia contact surface 355 of the tube wall 359. Based upon this conventional testing it seems very likely that nitriding of mild steel tubes when subjected to the working fluid of a Kalina cycle power generation system could result in such degradation to the structural integrity of the tube that the tube would be unable to withstand the required loading of, for example, the boiler walls over the desired design life of a practical power generation system. Accordingly, it is generally accepted, based upon conventional testing, that because of nitriding mild steel tubes will be unsuitable for practical Kalina cycle boilers.